The solubility of oxygen in the blood

The major respiratory function of the lungs is to add oxygen to the blood and to remove carbon dioxide from it. Thus the measurement of the concentration of these gases in the arterial blood leaving the lungs, combined with a knowledge of the partial pressure of oxygen in the inspired air (approximately 147 torr at 37°C), allows an assessment of the gas exchanging function of the lungs.

The solubility of oxygen in the blood is dependent upon the concentration of haemo globin, each gram of which can combine with 1.34 cm3 of oxygen at 37°C, and upon the presence of other ligands which combine with haemoglobin and affect oxygen binding. The oxygen saturation, SO , of a particular blood sample, which determ2ines the colour of the blood, is defined by the ratio of the oxygen concentration in the blood sample to the oxygen concentration when that blood is fully saturated (i.e. the oxygen capacity of the blood). Defined in this manner, it is clear that So for an anaemic patient, where there is a low2 haemoglobin content, may be the same as that for a patient with polycythaemia, but the oxygen concentration of the blood would be much less in the anaemic patient.

The partial pressure, PO , of the oxygen in the blood (oxygen tension2) is related to SO by the oxygen dissociation curve (Fig. 2.11). The shape and position of this sigmoidal curve depend on the temperature, the hydrogen ion concentration and the concentration within the red cells of other ligands of haemoglobin

which may also bind to this molecule in addition to oxygen. The position of the curve is defined by the PO at 50% saturation, which is denoted as P50. An alternative method of plotting the data uses the logarithmic equation

Other ligands of the haemoglobin molecule apart from oxygen which can affect these values include 2,3-diphosphoglycerate, a byproduct within the Embden-Meyerhof glyco-lytic pathway in the red cell. This is normally present in equimolar concentrations to = n log PO (2.21) haemoglobin. Transfused blood stored in acid-

citrate dextrose, however, contains very little Plots of log[SO /(1 - SO)] against log PO are of this comPound, and a lowering of P50 is linear over most2 of the range with a gradient n. noted over several hours in patients receiving log

As seen from Fig. 2.11, the P50 value is affected by pH change. Oxygenation of the haemoglobin molecule releases hydrogen ions, i.e. oxygenated haemoglobin behaves as a stronger acid (proton donor) than reduced haemoglobin. The ratio of AP50 to A pH (where A refers to the change in the property) is referred to as the Bohr effect and normally has a value of 0.5. It is usual to correct the P50 value to a plasma pH of 7.4 (although the pH in the red blood cell is about 7.18).

Normal haemoglobin has a P50 of 3.4 kPa and n of 2.6-3.0 at pH 7.4. Values of both P50 and n are affected by genetic abnormalities in haemoglobin synthesis that alter the amino acid sequence. Over 190 such variants are known, with a wide range of P50 and n values.

massive blood transfusions. P50 is also affected by the presence of carbon monoxide, which may result from heavy smoking or endogenous haemolysis.